Various types of valves are available to be used across a wide range of applications, such as chemical processing, water control, petroleum refining, and fluid transport. Ball valves are a very popular choice for many of these applications because ball valves are reliable and simple to use. A ball valve in its simplest form comprises a housing or body having passages in each end of the body that can be placed in line with, and attached to, a pipe carrying a fluid. A ball having a cylindrical flow passage drilled through it is then placed in the valve body. The ball can be rotated inside the body so that fluid flows when the flow passage in the valve is in line with the passages in the valve body and with the pipe, and fluid does not flow when the passage through the ball is cross-wise to the passages in the valve body and the pipe (i.e., when the pipe “sees” only the solid sides of the ball). In this manner, a ball valve can be turned from completely closed to wide open simply by turning the ball one-quarter turn. This turn can be provided manually or by an actuator, such as an actuator that may be driven by a signal from an automatic, computerized control system or by manually adjusting a switch.
It is also common to produce a ball valve in which the “ball” element is merely a partial segment of a full ball. In particular, the ball may include only enough material so that the hole in the body is fully blocked on the upstream side of the body when the valve is full-shut. Generally, this ball segment is only on the upstream or intake side of the valve, as there is little need to provide a blocking surface on the downstream side where the fluid is not impinging. Although the ball segment need not extend back to the middle of the valve to provide a seal, the ball segment can be formed to have tabs or ears that extend backward to accept the shaft or shafts on which the ball rotates in the valve body. Thus, when the valve is opened, the segment swings to one side of the flow path, and fluid flows past the backside of the ball segment.
While ball valves can be turned on or off easily, they also can be turned partway on or partway off to provide controlled, throttled fluid flow. When the valve is full-open, the cross-section of the fluid path through the valve will be circular, as the circular flow passage in the valve aligns completely with the circular hole in the body. As the valve is closed and the side edge of the flow passage in the ball (or the curved edge of a ball segment's back side) approaches the side edge of the flow passage in the body, the area through which fluid may pass goes from a circle, to a football shape, and then to a narrow vertical sliver that is pointed at the top and bottom, and that fattens slightly at the middle. In theory, a ball valve has an equal percentage type control characteristic, in that equal increments of relative ball rotation should yield equal percentage increments of the relative flow coefficient. The decreasing width and length of the flow path as a ball valve is closed, however, causes the valve to throttle more quickly with each corresponding degree of rotation than it did when the valve was full-open. In addition, the small sliver can create turbulence and pressure drop as a result of the Carnot effect. As a result, it can be difficult to provide precise control at very low flows using a ball valve.
For a control ball valve, the leading edge of the ball can have a major influence on the amount of fluid flow per degree of angular rotation, particularly when the valve is opened a very small amount. The leading edge is the edge that closes against the valve body as the flow path is fully blocked, and that faces the oncoming fluid flow when the valve is opened. Some prior ball valves have provided a smoothed v-notch shape on the leading edge of the flow passage or ball segment to better control the fluid flow characteristics of the valve, particularly at low flows. Other contoured leading edges may also be provided, and a contoured leading edge may be provided in combination with a contoured trailing edge having a different profile from the leading edge. However, while a contoured leading edge shape generally provides a more controllable response characteristic at lower flow openings, it typically limits the amount of fluid flow when the valve is at a high-flow condition. Therefore, it is desirable to have a control ball valve that provides good control characteristics and good low-flow and good high-flow characteristics.